D. Chandrasekharam

Arsenic enrichment in groundwater of West Bengal, India: geochemical evidence for mobilization of As under reducing conditions

Abstract The mechanism of As release and source(s) of As has been investigated in a small part of a watershed in the Murshidabad district of West Bengal. Analyses include major ion and trace element concentrations, as well as O, H and S isotope ratios of groundwater, surface water and a thermal spring. The results indicate that all water samples belong to the Ca–HCO 3 type, except for the thermal spring which is of the Na–HCO 3 type. Shallow and deeper groundwaters have distinct hydrochemical features. High As contents were registered only in the deeper groundwater horizon. Factor analysis and the distribution pattern of major and trace elements indicate that As is present in the aquifer as a scavenged phase by Fe(III) and to a lesser extent by Mn(IV) phases. The release of As into the groundwater occurs gradually in successive stages, corresponding to the actual redox state in the aquifer. The main stage of As release is related to the bacterial reduction of Fe(III) to Fe(II) (i.e. to the simultaneous dissolution of Fe oxyhydroxides). Low redox conditions in highly polluted areas are indicated by low SO 4 concentration and high δ 34 S values. During bacterial SO 4 reduction, residual SO 4 in groundwater is depleted in the lighter S isotope ( 32 S). However, the cause of the gradual decrease of the redox state in the groundwater is still not well understood.

Structure and evolution of the western continental margin of India deduced from gravity, seismic, geomagnetic and geochronological studies *

Abstract Available geochronological, gravity, seismic and geomagnetic data on the western continental margin of India between 8–20°N are presented. Based on the data three models describing the evolution of the crustal structure are developed. Model 1 describes the evolution of the crustal structure along 19°N. The main basic igneous activity that accompanied the tectonic events since the Late Cretaceous resulted in the formation of rifts along the coast thus giving rise to the present day structure. The acid volcanic event that preceded the above activity at about 93 Ma ago is responsible for the observed crustal structure along 13°30′N shown in model 2. This event resulted in the formation of the St. Mary group of islands along this latitude. Counterclockwise rotation of the Indian plate during the Tertiary period caused rifting of the continental blocks giving rise to the Laccadive-Kerala graben bounded by the Chagos-Laccadive ridge on the west and the west coast fault on the east. Vertical movements of the rifted blocks promoted the deposition of both continental and marine sediments on the coast west of the west coast fault. This tectonic event was accompanied by upwarping of the mantle-crust thus giving rise to the present day structure shown in model 3.

Origin and evolution of ‘intracratonic’ thermal fluids from central-western peninsular India

The chemical and isotopic composition of several thermal springs and associated gas phases in a large sector of central-western peninsular India has been investigated. Such springs have meteoric isotopic signature and emerge, after very well developed convective circulation at depth, along important tectonic structures such as the Son–Narmada–Tapti rift zone and the West Coast Fault. Chemical components in both gas and liquid phases and geothermometric estimations suggest that such springs are not related to the presence of any active hydrothermal systems at shallow depth in any of the studied areas. The hottest convective water emerges at Tattapani at near boiling point for water at atmospheric pressure (>90°C) in association with an N2-rich gas phase of clear meteoric signature. Since such fluids do not carry any corrosive components, they could be conveniently exploited for industrial purposes, such as drying processes. From a tectonic point of view, the presence of thermal emergences scattered in a wide area along geologically well defined structures, which also generate frequent moderate earthquakes, suggests that such structures are active. Although the isotopic composition of thermal springs points to a meteoric origin, their feeding aquifers are not topographically driven as in most active Alpine orogenic belts. The relative high quantity of total helium in the associated gas phase suggests also that they are really deep, old, long circulating waters. We propose for such waters the term ‘intracratonic thermal waters’ since the isotopic signature of He in the gas phase does not show any release of primordial 3He in any of the areas of spring emergence. Based on the quite low 3He/4He ratio in the gas phase we suggest also that, in spite of its morphological shape, the Narmada–Son–Tapti rift zone cutting the Indian subcontinent in two is more related to paleo-suture rather than to a mid-continental rift system.

Elemental and Nd-Sr-Pb isotope geochemistry of flows and dikes from the Tapi rift, Deccan flood basalt province, India

Abstract The Deccan Traps are a large rift-associated continental flood basalt province in India, parts of which have been studied extensively in terms of geochemistry, palaeomagnetism and stratigraphy. However, the basalts of the Tapi rift in the central part of the province have been little-studied thus far. Two ENE–WSW-trending tectonic inliers of the Deccan basalts in this region, forming ridges rising from younger alluvium, are made up of basalt flows profusely intruded by basaltic dikes. Both of these ridges lie along a single lineament, although they are not physically continuous. The flows are aphyric, plagioclase-phyric and giant-plagioclase basalts, and the dikes are aphyric or plagioclase-phyric. We consider the two inliers to have been originally continuous, from the presence of bouldery remnants of a major dioritic gabbro dike along both. Samples of this dike from both ridges have previously yielded typical Deccan ages of 65.6±0.5 Ma and 65.6±0.6 Ma by the 40 Ar – 39 Ar incremental heating technique. Initial 87 Sr / 86 Sr ratios and eNd(t) values, and present-day Pb isotopic ratios of most dikes indicate that they are isotopically similar to lavas of the Mahabaleshwar and Panhala Formations of the Western Ghats, about 450 km to the south. Their mantle-normalized trace element patterns have small Pb and Ba peaks. One dike has a strong Bushe Formation affinity and a Nd–Sr isotopic composition more extreme than that of any other Deccan rock yet sampled, with eNd(t)=−20.2 and ( 87 Sr / 86 Sr ) t =0.72315 . Its mantle-normalized element pattern shows large Pb, Th and U peaks and large Nb–Ta troughs. Its elemental and isotopic chemistry reflects substantial continental contamination. The flows cut by the Mahabaleshwar-type dikes are isotopically similar to the Poladpur Formation lavas of the Western Ghats. Their mantle-normalized element patterns show modest peaks at Rb, Ba and Pb and rather low Nb and Ta relative to La, indicating that they have been contaminated to intermediate degrees. The mantle-normalized element patterns of all the flows and dikes show enrichment in the light rare-earth elements, with small or no Eu anomalies. The entire flow-dike sequence is similar to the Wai Subgroup of the Western Ghats, in terms of its elemental and isotopic chemistry and stratigraphic relationships. Wai Subgroup-like lavas (i.e., some of the younger magma types originally identified from the southern part of the Western Ghats) are previously known from the central, northern and northeastern Deccan, and many have been thought to be far-travelled flows erupted in the southwestern Deccan. Although at least the dikes, and probably the giant plagioclase basalt flows of our study area, are locally generated and emplaced, our new data extend the known outcrop area of these widespread magma types substantially, and these magma types indeed appear to have a nearly province-wide distribution.

Influence of traditional agricultural practices on mobilization of arsenic from sediments to groundwater in Bengal delta.

In the wake of the idea that surface derived dissolved organic carbon (DOC) plays an important role in the mobilization of arsenic (As) from sediments to groundwater and may provide a vital tool in understanding the mechanism of As contamination (mobilization/fixation) in Bengal delta; a study has been carried out. Agricultural fields that mainly cultivate rice (paddy fields) leave significantly large quantities of organic matter/organic carbon on the surface of Bengal delta which during monsoon starts decomposing and produces DOC. The DOC thus produced percolates down with rain water and mobilizes As from the sediments. Investigations on sediment samples collected from a paddy field clearly indicate that As coming on to the surface along with the irrigation water accumulates itself in the top few meters of sediment profile. The column experiments carried out on a 9 m deep sediment profile demonstrates that DOC has a strong potential to mobilize As from the paddy fields and the water recharging the aquifer through such agricultural fields contain As well above the WHO limit thus contaminating the shallow groundwater. Experiment also demonstrates that decay of organic matter induces reducing condition in the sediments. Progressively increasing reducing conditions not only prevent the adsorption of As on mineral surfaces but also cause mobilization of previously sorbed arsenic. There seems to be a cyclic pattern where As from deeper levels comes to the surface with irrigational water, accumulates itself in the sediments, and ultimately moves down to the shallow groundwater. The extensive and continual exploitation of intermediate/deep groundwater accelerates this cyclic process and helps in the movement of shallow contaminated groundwater to the deeper levels.

Plume-rift interaction in the Deccan volcanic province

Abstract It is widely accepted that the grand volcanic episode of the Deccan in India was a consequence of the passage of the northerly drifting Indian subcontinent over the Reunion starting plume in the Late Cretaceous. This plume also produced the three-rift Cambay triple junction, the three arms being the West Coast graben belt, the Narmada-Tapi rift zone and the Cambay rift. Deccan-related alkalic magmatism both preceded and followed the main tholeiitic phase of ∼ 65 Ma ago by about 3 MY. The uniformly tholeiitic thick basalt sequence of the Western Ghats was derived from direct melting of the plume head, but the tholeiitic-alkalic magmatism of the three rift belts was due to melting of the hydrous lithospheric mantle due to variable amounts of rifting. Any lithospheric loading mechanism for generating post-tholeiite alkalic lavas does not seem feasible for the Deccan; nor can it explain the pre-tholeiite alkaline magmatism in the Deccan. Similarly, conductive heating of the lithosphere by the plume cannot have generated the alkalic melts. Some amount of actual rifting did occur. Direct melt supply from the plume is not necessary for magmatism in the rifts, though it seems to have occurred in some cases. Our documentation of indigenous magmatism for the three rift belts calls into question any attempts at inferring the size of the plume head from synchroneity in geochronologic data alone, and we negate the possibility or necessity of large-distance transport of magmas from the plume through the lithosphere, to feed the rifts. Recent evidence indicates that alkalic magmatism north of the main Deccan outcrop preceded the main tholeiite phase by ∼ 3 MY. The Cambay graben was active 3.5 MY before the main tholeiite phase. As the alkalic complexes at the northern end of this graben are 68.5 Ma in age (so far the oldest well-proven Deccan-related rocks), they are the first expression of the Reunion plume, and the borehole picrites from the triple junction area, until now conventionally regarded as the oldest, can no longer be considered so. Accurate radiometric dating of these picrites is therefore much needed.

Geothermal energy resources for developing countries

This text aims to be a driving force for an economically sound and sustainable development of developing countries. It looks at the provision of geothermal energy within the framework of sustainable energy development for power generation, rural electrification and so forth.

Geochemistry of Tattapani thermal springs, madhya Pradesh, India—field and experimental investigations

The chemical composition of Tattapani thermal springs, together with the experimental results on host rock-meteoric water interaction at 100°C, indicate dissolution of K-feldspars and albite during the initial stages of reaction. These phases control the chemical composition of the thermal springs. The estimated reservoir temperatures range from 205 to 217°C. Assuming an average thermal gradient of 80°C/km, the minimum depth of the reservoir is estimated to be around 3 km.

Deccan Traps Flood Basalt Province: An Evaluation of the Thermochemical Plume Model

The Deccan volcanic event occurred when India was situated at the present location of Reunion Island in the Indian Ocean. The cause, duration of eruptive pulses of this major volcanic event and its impact on the global climate are controversial. Plume versus non-plume hypothesis for the origin of Deccan melting anomaly is evaluated here based on a review of geochemical and limited geophysical criteria. We know that the most primitive Deccan magmas were picritic in composition that equilibrated in the mantle at about 1,550°C (±25) and 2.5 (±0.3) GPa. These magmas were generated from a large plume. We find that much of the differentiated and contaminated appearing basalts could not have been produced from such picritic magmas, and suggest that such tholeiites are a product of melts derived from the plume, the subcontinental lithospheric peridotites, and from ancient orogenic-type eclogitic blocks embedded within the continental lithosphere.

Geochemistry, geothermics and relationship to active tectonics of Gujarat and Rajasthan thermal discharges, India

Abstract Most thermal spring discharges of Rajasthan and Gujarat in northwestern India have been sampled and analysed for major and trace elements in both the liquid and associated gas phase, and for 18 O/ 16 O, D/H (in water), 3 He/ 4 He and 13 C/ 12 C in CO 2 (in gas) isotopic ratios. Most thermal springs in Rajasthan are tightly associated to the several regional NE–SW strike-slip faults bordering NE–SW ridges formed by Archaean rocks at the contact with Quaternary alluvial and aeolian sedimentary deposits of the Rajasthan desert. Their Ca–HCO 3 immature character and isotopic composition reveals: (1) meteoric origin, (2) relatively shallow circulation inside the crystalline Archaean formations, (3) very fast rise along faults, and (4) deep storage temperatures of the same order of magnitude as discharging temperatures (50–90°C). Thermal spring discharges in Gujarat are spread over a larger area than in Rajasthan and are associated both with the NNW–SSE fault systems bordering the Cambay basin and the ENE–WSW strike-slip fault systems in the Saurashtra province, west of the Cambay basin. Chemical and isotopic compositions of springs in both areas suggest a meteoric origin of deep thermal waters. They mix with fresh or fossil seawater entering the thermal paths of the spring systems through both the fault systems bordering the Cambay basin, as well as faults and fractures occurring inside the permeable Deccan Basalt Trap in the Saurashtra province. The associated gas phase, at all sampled sites, shows similar features: (1) it is dominated by the presence of atmospheric components (N 2 and Ar), (2) it has high crustal 4 He enrichment, (3) it shows crustal 3 He/ 4 He signature, (4) it has low CO 2 concentration, and (5) the only analysed sample for 13 C/ 12 C isotopic ratio in CO 2 suggests that CO 2 has a strong, isotopically light organic imprint. All these features and chemical geothermometer estimates of spring waters suggest that any active deep hydrothermal system at the base of the Cambay basin (about 2000–3000 m) has low-to-medium enthalpy characteristics, with maximum deep temperature in the storage zone of about 150°C. In a regional overview, both thermal emergences of Rajasthan and Gujarat could be controlled by the counter-clockwise rotation of India.